This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The goal of this project is to expand the population of regulatory T cells in young rhesus macaques and to assess the effect of such expansion on immune responses, such as SIV to vaccines. Regulatory T cells are T cells that inhibit immune responses mounted by other T cells. We have previously found that young macaques have more of these cells than older macaques, so we hypothesized that drugs acting to expand regulatory T cells would be more effective in younger animals. The project has two specific aims: 1) assess whether a high frequency of T-regs observed in neonatal macaques can be maintained or increased by administration of IL-2 and G-CSF and 2) determine the impact of an expanded regulatory cell population on immune responses.